As is well-known, one of the most used techniques in the mobile radio communication systems is the W-CDMA (Wideband-Code Division Multiple Access) technique, by virtue of its high spectral efficiency as compared to other multiple access techniques. Within said technique and in particular in the FDD (Frequency Division Duplexing) mode different solutions have been adopted to increase system capacity. Among said solutions it is worth mentioning the coherent tracking of the signal received by the radio mobile station or mobile terminal.
This type of signal tracking requires a very accurate estimation of the transfer function of the transmission channel, referenced to in the sequel as “channel estimation”, in the presence of fading and multiple reflections due to user's motion in the covering area, without giving any details about a user's displacement velocity.
Like other system functions, also the channel estimation function may be assigned to processing devices (for example, DSPs, microcontrollers, etc) and in such a case this is achieved by means of software or through dedicated devices (for example FPGAs, ASICs, etc.) and therefore the implementation modality is mainly hardware based. During the definition of the telecommunications system architecture, the design engineer has to find out the best distribution of the different tasks among programmed devices and wired devices.
Thus, the different architectures achievable must then be analysed in terms of computational burden for a DSP device, traffic volume over the bus dedicated to data transfer, and the optimum distribution among hardware and software resources.
Considering by way of an example a complex system such as a modem for a UMTS base station or mobile terminal, it is evident that the communication bus between the DSP device and the hardware resources acts as a bottleneck of the system. For instance, as to a radio base station, the load of the bus becomes greater and greater as the number of users increases, and when the functions involved require the transfer between DSP and hardware resources of considerable bulks of data.
Channel estimation is one of said functionalities, since it has to be repeated for any fingers (sub-channels) of each individual user.
Let us consider the worst case in which the maximum number of users Nu=128 are communicating within a cell and each user has a finger number Nf=8 for each Rake receiver (a receiver which is typical for example of UMTS base stations). The quantity of data to be transferred for each finger in a time slot (time interval) of duration Ts=666 μs may be estimated to be equal to a number of bits NB=400. As a consequence, there will be a considerable data stream over the bus, namely:Nb·Nf·Nu/Ts=400·8·128/666·10−6=615 Mbit/s 
As of today, various algorithms capable of performing the channel estimation are known; among them there are for instance those described in the document EP 0 912 019, wherein two different interpolation methods, the one of linear type, the other through Kalman filters, are used, the method described in the document EP 1 032 168, wherein the interpolation is performed with the method of Lagrange's polynomials, and that described in the document U.S. Pat. No. 5,886,911, in which use is made of the bi-section method.
The solutions described in the above documents have however some drawbacks and are not directly applicable to UMTS systems, such as the bi-section method, or are extremely complex for an implementation of hardware type.
Taking into account as a matter of fact, the considerable data stream already present on the bus of a base station, it should be preferred not to further overload such a bus, since this would require complex interpolation operations within the estimation of the channel, such as the operations necessary to perform an interpolation by the method of Lagrange's polynomials.
The device can be fully implemented through hardware and therefore can be easily integrated into a Rake receiver for base stations, keeping the same performance as of the software solutions based on DSP processors. Since no data transfer with the DSP processor is required, the communication bus is left free for other tasks.
A particular subject matter of the present invention are a method and a device for the estimation of the transfer function of a transmission channel, as described in the appended claims.